1. Field of the Invention
The present invention relates to boat propulsion, and more specifically to a propeller positioning system which constrains the propeller to follow a path generally parallel to the bottom surface of the boat.
2. Description of the Related Art
Trim tabs have been used to change the “attitude” of a boat. Attitude is the angle of the boat relative to the water surface, and changes under different operating conditions. The attitude of a boat relative to the water surface has a profound effect on the speed and efficiency of the boat. Attitude is usually discussed in terms of the nose up/nose down adjustment of the boat, and is sometimes called the trim angle. The term “trim angle” often leads to ambiguity as to whether it is the angle of the boat, or the tabs, or the outdrive being discussed. The present disclosure attempts to be more specific in discussing trim angle.
Trim tabs are usually fastened to the boat at or near the stern and on the transom or on the bottom of the hull. The transom generally forms the rearmost portion of the stern, such as the generally flat and vertical rearward end of the chassis of the boat. When underway, water rushes under the boat, causing the rear of the boat to be deflected up or down by the trim tabs. Pushing the trim tab down deflects the departing water downward to boost the rear of the boat up into the air slightly, thus bringing the bow of the boat down. Pulling the trim tab up is intended to pull the rear of the boat down and to bring the bow up. Thus, “down on the tabs” means “down on the bow” and, conversely, “up on the tabs” means “up on the bow”. It is noted, however, that the “up on the tabs” operation of prior art trim tabs has limited effectiveness. Attitude may sometimes be discussed in terms of the left or right lean of a boat under way. Leaning may be due to propeller torque, uneven weight distribution, or cornering. Trim tabs may also be used correct this leaning.
Although trim tabs are an appurtenance to the hull, they serve to modify the shape of the planing surface and, therefore, from the perspective of hydrodynamics of the boat planing on the water, it is immaterial whether the tabs are considered as part of the hull or an appurtenance.
Prior art trim tabs are only somewhat effective in changing the attitude of the boat. Early prior art trim tabs were hinged where they joined the hull of the boat and usually were a rigid flat plate essentially parallel to the bottom surface of the hull. This flat plate could swing up or down several degrees via mechanical means. A major deficiency of flat plate hinged tabs is that the up tab position causes an abrupt change in the contour of the surface running on the water. This abrupt change causes flow separation at the hinge point. As with any airfoil, flow separation causes loss of lift. The hinged flat plate is simply a crude airfoil with poor lift to drag ratio and is not very successful at raising the nose of the boat. Hinge type tabs in the down position will lower the bow of the boat, but have a poor lift to drag ratio and tend to impose excessive drag in order to generate an equivalent amount of lift of present invention.
More recent prior art trim tabs are of a bending flat plate type whereby the tab is a resilient plate of uniform thickness and stiffness. A flat plate is attached solidly in a cantilever fashion to the boat hull and does not use a hinged joint but rather relies on the bending of the flat plate slightly up or down to generate a somewhat better, but still deficient, approximation of an airfoil. The bending flat plate trim tabs were flexed down and up by the boat operator to add hook or rocker as desired. Hook is usually caused by a concave surface on the bottom of the boat, when viewed from below the boat, which tends to lower the bow while underway. Rocker is usually caused by a convex surface on the bottom of the boat, when viewed from below the boat, which tends to raise the bow of the boat while underway. The bending flat plate trim tabs were slightly more effective than hinged type plates. Although somewhat superior to hinged plate designs, the bending flat plate also has excessive drag for the amount of lift generated. Bending flat plate trim tabs are somewhat better than the hinged type in that the problematic abrupt change of angle of the hinge type is softened. This curved surface method decreases the tendency of flow separation, but uses a plate of constant flexural stiffness, so that the curvature is fairly localized and diminishes as the water moves away from the area of attachment of the plate to the hull.
The attitude of a boat may also be changed by changing the thrust angle of the propeller as ordinarily found on outboard motors and on inboard/outboard propulsion systems. This function is normally controlled by a “trim” switch, which is part of an electro-hydraulic system. This method of changing the attitude of the boat and lifting or depressing the stern of the boat by changing the thrust angle of the propulsion is not as efficient as optimizing the thrust vector to be essentially parallel to the direction of travel and then adjusting the hull geometry to optimize the hydrodynamic lift to drag ratio.
Surface piercing drives are well known and are usually referred to as surface drives. One common type is mounted to project out the stern of the boat so that the propeller is located at the far aft or rearward end of an extended quasi-cylindrical thrust tube. Surface drives are designed to allow only the lower half of the propeller to be in the water at design operating conditions. In other words, surface drives only operate on the surface of the water. The depth to which the propeller is actually immersed in the water at any given operating speed has a major effect on the resistive torque load imposed on the engine. Therefore, it is desirable to vary the depth of immersion as needed but to also be able to closely control this depth to something less than half of the propeller diameter under all normal operating conditions. The propeller blade immersion or depth of partial immersion of the propeller is referred to herein as “bite”.
Surface piercing propeller drives are known to be very efficient propulsion systems. The most versatile surface drives have a means for thrust vectoring by changing the horizontal and vertical alignment of the propeller shaft with respect to the stern of the boat. This realignment is accomplished via a universal joint where the drive shaft exits the hull near the transom and connects to the propeller shaft. Mechanical means, usually of electrohydraulic cylinders push and pull the thrust tube in a preferred horizontal sweep or a vertical sweep. The horizontal sweep controls the steering of the boat, and the vertical sweep controls the trim of the boat. The steering sweep is somewhat effective, but the vertical sweep for controlling trim is rather deficient.
Surface drives have two primary advantages. The first advantage is that they have very little drag as only the propeller and skeg are in the water. The skeg is a fin that is affixed to the underside of the drive at a location forward of the propeller and that has a leading edge which sweeps generally downward and rearward. The skeg is supposed to protect the propeller from objects in the water and conversely to protect objects from the propeller. Neither is very successful. The skeg is also intended to contribute to the steering, but because a surface drive has a steerable thrust vector, the skeg has only a minimal contribution to the steering, especially when under power.
The skeg imposes undesirable drag and also slices a groove in the water as the water approaches the propeller blades. In this manner, the skeg causes a major disruption in the homogeneous flow field of water into which the blades are progressing as the propeller screws itself into the water. Although there is a performance penalty due to the drag forces of the skeg in the water, there is an added performance penalty over and above that simple drag force due to the field disruption and aeration of the water ahead of the propeller. This is why many competitive race boats place the rudder, if necessary for steering, off to the side of the propeller so as not to interfere with the flow field in proximity of the propeller.
The second primary advantage of surface drives is that the propeller is designed to be in a ventilating mode at normal operating conditions. In contrast, most inboards and outdrives with submerged propellers use nonventilating propellers and hence they have a maximum theoretical speed limit before the propeller goes into the cavitation mode and loses its grip. A propeller designed to be nonventilating does not work well in a ventilating mode and conversely, a propeller designed to operate in the ventilating mode, which is sometimes referred to as “hyperventilating” mode, does not work well in a nonventilating mode. There is no maximum speed for a ventilating or hyperventilating propeller as there is for a nonventilating propeller.
Surface drives have a disadvantage of not backing up very well. This is caused by the propeller back wash impinging on the transom of the boat and nullifying the reversing thrust. In contrast, non surface drive boats have their propellers immersed deep enough to let the reverse propeller wash carry under the hull. Surface drives also have a disadvantage of presenting an exposed propeller at the surface of the water such that a passenger could fall into contact with the blades more easily than with submerged propeller type of drives.
With ordinary full immersion outdrives and outboard motors, the trim effect on the boat can be accomplished by tilting the lower unit as “trim out” or “trim in”, hence tilting the thrust vector up or down, to raise or lower the nose of the boat. However, as previously discussed, the vertical thrust vectoring of the propulsion system is inefficient compared to the preferred trimming of the hull through the use of trim tabs. Using the up trim on a surface drive has a very weak effect on the attitude of the boat and usually only tends to pull the propeller too far out of the water causing the engine to overspeed. Using the down trim immerses the propeller too far into the water causing the engine to slow down due to overload. In general, the up and down trim of a surface drive affects the bite of the propeller much more than it affects the attitude of the boat.
Controlling the bite of the propeller has the same effect as that of changing the pitch of the propeller, or changing the propeller diameter, or even changing the gear ratio of the transmission. The difficult part of controlling this bite is that the water depth on the surfacing propeller is influenced by wave action, boat bounce, cornering, or speed changes. Controlling the bite is advantageous in that it is like having an infinitely adjustable transmission to match the engine performance to the propeller load. However, sporadic changes in bite are not desirable because it is hard on the drive train, it makes the boat difficult to control, and it may be dangerous to the occupants.
Another disadvantage of surface drives is that they tend to crawl sideways due to having only the lower half of the propeller in the water, which thus imparts a net sideways force on the propeller. In other words, the propeller tries to crawl the back of the boat to the side. Side crawl pulls the thrust tube to one side and if the steering wheel is released, the boat turns sharply to that side, but if the wheel is held straight ahead, the boat veers to the other side following what is known as the “crabbing angle”. If the boat has two surface drives mounted on the transom in the ordinary fashion, and they are rotating in opposite directions, the side forces tend to cancel and the boat does not crawl to the side. It is important to note, however, that under normal conditions such as wave action, boat bounce, cornering, and speed changes, one propeller can dig into the water deeper than its twin and therefore cause the boat to momentarily crawl sideways. This side motion can be chronic, unpredictable, and fatiguing to the driver.
Another disadvantage of surface drives is that the boat tends to struggle to get up on plane. Because the torque output of an engine is low at low revolutions per minute (RPM) and because the propeller is fully immersed, or “flooded”, at low speeds, surface drive boats often have a difficult time getting over “hump speed”, which is that speed when the boat actually starts to plane and the drag forces tend to decrease. The previous ways to overcome this bogging down effect is to use a multi-speed transmission, or to vastly overpower the boat with a very powerful motor.
Another disadvantage with surface drive boats is they may tend to “blow over” at high speeds. Blow over means that the boat goes airborne, nose up, and does a complete back flip. Blow over starts with too much nose up, then the hull starts “kiting”, and the propeller continues to run up under the boat, and it flips over. Although other high speed boats also blow over, the surface drive may be worse in that, as the bow of the boat starts to rise up, the extended propeller is dunked under the water surface as the boat tends to rise up on its bottom surface at the stern thus adding to the blow over condition.
Prior boat designs do not address the changing bite associated with wave action, or hull bounce, or changes in speed, or the fact that the plane of water leaving the bottom of the craft when turning is not in the same plane as when the craft is traveling in a straight line. Prior designs do not address the uncontrolled bite problem. Rather, the driver is forced to make continual up and down changes on the prior art hydraulic cylinder, which is attached to the thrust tube, to try to compensate for these continually changing operating conditions.